An example of a conventional resistance spot welding machine is a machine disposed along a production line for automobile bodies, the machine having a robot arm which is moved between a retracted position and one or more spot welding positions as each automobile body on the assembly line passes the machine. Large bursts of electrical current are required to carry out resistance spot welding, and thus a welding cable supplying electricity to the robot arm must have one or more copper conductor ropes of significant diameter. These conductors tend to become heated as a result of the large currents flowing through them, and therefore it is conventional to provide a tubular hoselike cover which extends the length of the cable, and to pump a cooling fluid through the cable from one end to the other during use of the cable.
The hoselike cable covers are typically rather stiff, and they tend to produce significant resistance to torsional movement of the cable, namely twisting of the cable about its lengthwise axis. Since a robot arm must be able to carry out a wide range of movements in order to access hard-to-reach welding locations, there are many applications where a welding cable must be capable of carrying out a reasonable degree of torsional movement without exerting excessive resistance to such movement onto the robot arm. Consequently, for different existing robot arms, it has become customary to specify for each arm the maximum resistance which an associated welding cable of a specified length is permitted to offer to a specified amount of torsional movement, because if the cable exerts resistive forces in excess of this amount on the robot arm, the design parameters of the robot arm may be exceeded so that the arm either fails to function and/or is physically damaged.
Due to the stiffness inherently present in the welding cable covers conventionally provided to facilitate cable protection and fluid cooling, the resistance to torsional movement in most existing welding cable designs is typically very close to the specified maximums, which means that each cable manufactured must be carefully tested in order to ensure that it in fact meets the specifications. Further, even as to cables which do meet the specifications, the forces exerted on the tubular cable cover in applications which involve a significant amount of repetitive torsional cable movement tend to lead to cover failure and thus a reduction in the average useful lifetime of the cable.
A further problem exists in conventional cables of this type which have more than one electrical conductor. In particular, a conventional cable may have two conductors, each of which includes two copper conductor ropes. Thus, there are four conductors extending through the cable, and an insulating element is provided to prevent them from touching each other and creating an electrical short. In the conventional cable, this insulating element has the cross-sectional shape of a cross, including an elongate central portion with four walls extending outwardly in respective radial directions, each wall being disposed between a respective pair of the conductors. Due to the fact that large bursts of current pass through these welding cables, the cables tend to "kick", jump and twist in response to the electrical current. This, coupled with torsional cable movement in applications requiring torsional movement, sometimes causes a portion of one radial wall to shift in position so that the conductors on opposite sides of it can touch each other, thereby creating an electrical short which renders the cable useless and necessitates it replacement, resulting in both downtime for the assembly line and also additional cost for the replacement cable.
An object of the invention is therefore to provide a fluid-cooled welding cable with a cover which presents minimal resistance to torsional cable movement.
A further object of the invention is to provide a multi-conductor welding cable having an arrangement which reliably maintains the conductors in electrical isolation from each other despite the kicking, jumping and twisting which typically accompanies conduction of current bursts through the cable.